Effects of heavy metals on the reproduction of Rasbora dandia

Abstract

Aquatic environments are under the constant threat of toxicants from various anthropogenic sources. Heavy metals are one of the major pollutants of aquatic ecosystems all over the world. With persistence, high toxicity and property of bioaccumulation, heavy metals are of great concern among various pollutants. Mercury and cadmium are two highly toxic heavy metals with no known biological role.Wild fishes are a group of organisms worst affected by aquatic pollution. Fishes are significant part of almost all aquatic biota and the taxonomic group which suffers a major threat from the impact of pollutant contaminants in their environment. Reproduction is one of the fundamental processes of life which ensures the continuity of the species. Any environmental factor which impairs reproduction directly or indirectly will contribute to species extinction. Fishes are very suitable test group for studying the impact of aquatic pollutants on the reproductive function of organisms. The present study investigates the effects of heavy metals, mercury and cadmium, on the reproduction of the native fresh water teleost fish, Rasbora dandia.In order to confirm taxonomic status of R. dandia with molecular evidence, mitochondrial cytochrome oxidase sub unit I gene (COI) study has been conducted. The BLAST search using the COI sequence of R. dandia revealed that the sequence obtained in this study was novel (GenBank Accession No. KP742982). Phylogeny analysis using COI gene sequence provides molecular evidence to taxonomic identity of the species, R. dandia.As the present investigation is focused on the toxic impact of heavy metals on fish reproduction, reproductive biology of the test fish, R. dandia is elucidated. R. dandia is a seasonally breeding fish with a prolonged breeding season extending from June to November. Five maturity stages were identified in the gonads during the reproductive cycle of the fish: immature, maturing, mature, ripe and spent. Induced breeding of R. dandia was found successful with HCG at a dosage of 5000 IU (male) and 10,000 IU (female) per kg body weight of fish.Acute toxicity studies are the essential data requirement for proceeding further investigation in ecotoxicological studies and median lethal concentration of mercury and cadmium was assessed. 96 hours LC 50 of R. dandia for mercury and cadmium was found to be 133.30 ppb and 16.91 ppm respectively. Bioaccumulation is a vicious property of heavy metals which exacerbate their impact on the ecosystem. Bioaccumulation of mercury and cadmium in the gonads of test fish, R. dandia was studied. In bioaccumulation study, mercury and cadmium were undetected in control samples. Accumulation of cadmium increased as exposure period proceeds and both testes and ovaries exhibited same pattern. Mercury progressively accumulated in the testes and ovaries up to 20 days of exposure period. While ovary exhibited increased mercury accumulation at 30 days, testes showed no further increase in accumulation from 20th day.Effect of heavy metals in the sexually mature gonads of R. dandia is investigated in the present study. GSI of sexually mature R. dandia was found to be affected by mercury and cadmium exposure. Reduced spermatozoa, proliferation of interstitial tissue, disorganised bundles and appearance of vacant spaces were the major histopathological changes occurred in the mature testes of R. dandia under mercury exposure. Dilated blood vessels, reduction in spermatozoa and detachment of basement membrane were the significant histopathological changes occurred in the mature testes of R. dandia under cadmium exposure. Mass atresia occurred in the mature ovary of R. dandia under mercury exposure. Fibrotic tissue appeared in the areas where oocytes disappeared. Ripe ovaries transformed in to shrunken immature ovaries with histopathological alterations under mercury exposure. Appearance of areas with mass cellular atresia was the main histopathological alteration occurred in the mature ovary of R. dandia under cadmium exposure.In this study, the effect of heavy metals, mercury and cadmium is evaluated on the gonadal recrudescence of R. dandia. Heavy metal treated recrudescent fishes showed lowered GSI compared to control recrudescent fishes at the end of exposure period. At the end of experimental period which conducted during gonad development season, gonads of control fishes became sexually mature with abundant ripe spermatozoa. Ripe spermatozoa were limited and early stages of spermatogenesis were prominent in mercury and cadmium treated male fishes. Female control fishes showed sexually mature ovary with abundance of mature oocytes. Mass atresia of immature oocytes occurred in the ovary of cadmium treated fishes. Mature oocytes were reduced in cadmium exposed fishes. Atresia of mature oocyte also occurred along with mass atresia of immature oocytes in mercury exposed fishes.Effect of heavy metals during the embryonic development of R. dandia is also carried in the study. Heavy metal exposure during embryonic development induced various structural anomalies in the larvae of R. dandia. Vertebral abnormalities were common in all experimental groups of larvae. Mercury and cadmium affected the inflation of swim bladder in the larvae of R. dandia. Rudimentary eyes and enlarged yolk sac were prevalent among mercury exposed larvae. C-shaped larvae were common among cadmium exposed group. During the beginning of hatching, hatching rate is elevated among mercury and cadmium exposed larvae. Most of the larvae hatched with abnormal morphology when exposed to mercury and cadmium, whereas abnormal morphology is very rare among control group.The present study indicates that the reproductive phases of the fishes are vulnerable to mercury and cadmium toxicity. Impact of these heavy metals on the embryonic development indicates the formation of poor quality of progeny which may end up in reduced population size. The study indicates heavy metals may impair reproduction in fishes and this may lead to the decline in fish populations.